Coincidence type optical and electro-optical rangefinders derive target range by precisely measuring the angle of parallax subtended by the target of interest and the left and right hand entrance windows whose spacing determines the baselength of the rangefinder. This angle of parallax is measured when the target images formed from the target radiation entering the left and right hand entrance windows are aligned in coincidence within the users' eye, or measured on a photodetecting array. The angle of parallax may also be measured on a photodetecting array where the target images from the left and right hand entrance windows are not aligned into coincidence, but where their separation on a photodetecting array is precisely determined. The range of the target of interest will then be a function of the angle of parallax.
Coincidence type rangefinders have several advantages over other rangefinding methods and devices. Coincidence type optical rangefinders do not require retro-reflective mirrors or prisms, nor do they require a rod person or rod positioned at the location of the target of interest. Coincidence type rangefinders do not emit any potentially eye damaging radiation, or require significant electrical power as do some laser rangefinders.
However, since coincidence type optical rangefinders measure very precise angles of parallax, they may easily lose correct calibration for range if subjected to vibration, shock, or thermal expansion or contraction of optical or mechanical components. Any displacement of optical elements within the rangefinder can adversely affect ranging accuracy unless precautions have been made in the rangefinder design itself to minimize or eliminate these sources of error.
In the very early optical rangefinders, pentaprisms were used as first reflecting surfaces which, when used in a collimated stream of light, are insensitive to rotations about the normal axis. Canadian Patent No. 549,248 to Malinowski (1957) provides one example of a rangefinder utilizing two pentaprisms. The use of pentaprisms will reduce the loss of range calibration caused by certain displacements of the pentaprisms, but are ineffectual at reducing or eliminating loss of range calibration caused by slight displacement of other optical or mechanical components of the system. As such, even where pentaprisms are utilized in optical rangefinders, the devices must be re-calibrated regularly for range to insure reliable measurements. One distinct disadvantage to the use of pentaprisms within optical rangefinders is that they are massive compared with single prism or mirror reflectors of similar aperture.
Many previous designs of optical rangefinders utilize a light compensator to align the target images formed from left and right hand entrance windows within the user's eye. The light compensator consists of a glass wedge or wedges, or a mirror or prism reflector which, when adjusted, will vary the lateral angle of light in the left or right hand channels of the rangefinder. The closer the target of interest to the observer, the more the light compensator will need to be adjusted to align the target images. The degree of adjustment required is then a function of the angle of parallax of the target and the target range is, in turn, a function of the angle of parallax. The degree of adjustment of the light compensator is often measured by mechanically coupling the light compensator to a scale from which range may be read, or to a position sensor which derives data for the calculation and display of target range. An example of the latter may be found in U.S. patent #3,499,711 to Argyle (1970).
This mechanical coupling of the light compensator to a scale or position sensor can be a significant source of ranging errors. Deformation of mechanical parts associated with this coupling can contribute to ranging inaccuracies. These deformations may be caused by mechanical shock, wear, or thermal expansion or contraction of the coupling mechanism. Frequent range recalibration may temporarily correct the resultant ranging inaccuracies caused by some of these deformations, however if wear of mechanical components introduces backlash or separations, it will not be possible to correct by recalibration. For many optical rangefinders, range recalibration is an exacting and time consuming procedure requiring optimal conditions of visibility which may not exist at the required time.
Further examples of optical rangefinders which are susceptible to errors of this nature are described in patents such as U.S. Pat. No. 3,459,478 entitled, "Stadiametric Rangefinder Including a Transversely Movable Lens" to Marasco et alis, (1969), and in U.S. Pat. No. 4,886,346 entitled "Range-Finding Binocular", to Monroe, (1989).
Most prior art coincidence optical rangefinders depend on the resolving abilities of the human eye to align images to coincidence. Target selection however is a coordinated effort by both the human eye and brain. Therefore, prior art coincidence type optical rangefinders which alternatively utilize photodetector arrays to assist in the alignment of target images are ineffective where objects of different ranges than the target object of interest form images on the photodetector arrays. In this case, the photodetecting system does not have the benefit of the human brain to ignore targets in the fore or background which are not the target of interest. As a result, where this situation occurs, coincidence optical rangefinders using only photodetectors for target image detection cannot isolate the target of interest and will either not be capable of providing range information or the information will be erroneous. U.S. Pat. No. 3,663,105 to Anderson (1972), describes an electro-optical rangefinder using photodetectors which would be susceptible to this problem.
Additionally, coincidence optical rangefinders using photodetectors for target image detection may encounter light conditions which are lower than the dynamic range of the photodetecting components employed. Rangefinders of this type will therefore be non-functional under these conditions unless provided with a fall-back-to-visual alignment mode, whereby the human eye, which has a wide dynamic range, is employed for image detection and alignment. Prior art rangefinders which are strictly electro-optical in design and offer no fall-back-to-visual image alignment mode are described in articles such as one entitled, "Passive Stereoscopic Rangefinder", British Aerospace, Dynamics Group Bristol (1983), or as applied to auto-focus ranging systems such as described in U.S. Pat. No. 4,835,561 entitled, "Focus Detecting Device for Camera", to Matsui (1989), and in U.S. Pat. No. 4,831,405 entitled, "Auto-focus Arithmetic Device", to Hata et alis, (1989).
Attempts have also been made to design optical rangefinders using illuminated reference markers for measuring and correcting for mechanical aberrations. U.S. Pat. No. 4,071,772 to Leitz et alis (1978) describes such a apparatus utilizing spatial frequency filters which comprise a measurement structure such as an optical grating on which are produced the target object images. Light fluxes obtained from the interaction of the images with the measurement structure when there is a relative motion between this structure and the images are converted by means of a photo-electric receiver system.
As the measurement structure is oscillated back and forth in either the reference marker images, or the target scene images, the light intensity registered by the photodetectors will vary in time with the motion of the measurement structure or optical grating.
In the device which is the subject of the present invention, a fundamentally different system is used to measure the separation of either the reference marker images, or the target scene images. In the present invention, the separation of either the reference marker images, or the target scene images is measured directly, independent of any measurement structure or optical grating. In the most simple form of the present invention, the human eye is used to align the target scene images, and a single photodetector is used to sense the separation of the reference marker images and thereby determine the target angle of parallax.
One method for directly measuring the separation of either the reference marker images, or the target scene images is to convert the images directly into electrical signals, and then by computational cross-correlation, determine the degree of image separation.
Another fundamental difference between the present invention, and the Leitz invention, is that the present invention uses a channel merging/splitting means to split each of the reference markers, and direct both reference marker beams onto a minimum of one photodetector. In contrast, the Leitz invention uses two reflecting faces which do not merge the reference marker beams into a single beam, and therefore requires separate pathways for each marker beam to a minimum of two photodetectors. More important in terms of utility, is that as a channel merging/splitting means is not fundamental to the Leitz invention, a means for providing dual images of the target to the user's eye is not inherent to the design. Therefore a visual fall back mode is absent, and no means is provided where should target scene conditions, or light levels fall below the requirements of the photodetectors, the human eye may be used to align the target scene images into coincidence. Additionally, as no dual images are provided to the user's eye, in situations where additional targets other than the target of interest are imaged onto the measurement structure of the Leitz invention, erroneous range measurements may result should the additional target be located at ranges different from that of the target of interest. Additionally, the present invention allows, in an embodiment with dual photodetector arrays, for the two photodetector arrays to be referenced one to another allowing for minor shifts in the position of the photodetectors to be accounted for. This is only possible because the reference marker beams are split. Leitz does not disclose any splitting of the reference markers and thus cannot reference the photodetectors one to the other.
The primary advantages therefore of the present invention over the Leitz invention are: utility; as a fall-back-to-visual mode is incorporated into embodiments of the present invention, reliability; as no oscillating mechanical structure is required which may be subject to friction and wear, dependability; as the user may isolate the target of interest from other targets by initially visually a aligning the target of interest, and finally flexibility; as the most basic embodiment may be adapted to rangefinders which use visual alignment of the target scene images, as well as to rangefinders which use other photodetectors for sensing the alignment of the target scene images.
Finally, conventional coincidence type optical rangefinders provide permanent dual or split images of the target scene to the eye of the user. These dual or split images may be annoying if it is desirable to use the device simply for observation. Such is the obvious case for binocular embodiments of coincidence type optical rangefinders.